The present invention relates to a detection circuit, a physical quantity measurement circuit, a physical quantity measurement device (e.g., vibrating gyroscope), an electronic instrument, and the like.
A detection circuit (sensor circuit) that receives a current (charge) signal corresponding to a change in physical quantity (e.g., angular velocity or acceleration) generated by a physical quantity transducer (i.e., a converter that converts a change in physical quantity into an electrical signal) must amplify the input current (charge) signal by a high amplification factor using a first-stage current/voltage conversion amplifier (hereinafter may be referred to as “I/V amplifier”).
The detection sensitivity (S/N ratio) of the detection circuit (sensor circuit) is affected to a large extent by the performance of the first-stage I/V amplifier. Therefore, the manufacturer must elaborately evaluate the characteristics of the first-stage I/V amplifier in order to establish an appropriate design method. A product (detection circuit) with low detection sensitivity may be screened out based on the evaluation results.
Examples of the evaluation items for the first-stage I/V amplifier include whether or not the I/V amplifier functions as an amplifier circuit, whether or not the gain determined by the absolute value of a feedback resistor is an appropriate value, a check on the frequency characteristics of the I/V amplifier when the I/V amplifier includes a feedback resistor and a feedback capacitor (i.e., has frequency characteristics), and the like.
The following factor makes it difficult to evaluate the characteristics of the first-stage I/V amplifier. Specifically, since a current (charge) signal (alternating current) from a physical quantity transducer is very small, it is difficult to create an equivalent evaluation current signal when evaluating the first-stage I/V amplifier.
Therefore, when evaluating the characteristics of the first-stage I/V amplifier, it is necessary to employ a method in which an input resistor with a given resistance is connected to the first-stage I/V amplifier, and a voltage signal with a given amplitude is supplied to the input resistor.
The input evaluation voltage signal is converted into a small current signal (corresponding to an alternating current signal from a physical quantity transducer) through the input resistor. Therefore, the input evaluation voltage signal is a signal equivalent to the alternating current signal.
JP-A-9-138141 discloses a sensor circuit configured to convert a current detection signal into a voltage using a current/voltage conversion resistor, for example. In JP-A-9-138141, the current/voltage conversion resistor can be selectively connected to an input terminal of the sensor circuit. The resistor is connected/disconnected using an internal switch.
JP-A-8-61958 discloses a vibrating gyroscope inspection device, for example. In JP-A-8-61958, a vibrating gyroscope inspection signal (current signal) is input to a first-stage amplifier through an input resistor.
The inventors of the invention discovered the following problems.
(1) When connecting an external input resistor to an input terminal (pad) of a detection circuit (sensor circuit) when evaluating a first-stage I/V amplifier, the evaluation accuracy of the characteristics of the first-stage I/V amplifier decreases due to the addition of a parasitic capacitor of the external input resistor and a parasitic capacitor of a wire for connecting the external input resistor. In particular, since the resistance of a feedback resistor is increased in order to obtain a high gain, the first-stage I/V amplifier tends to be affected by the parasitic capacitor (i.e., the CR time constant changes due to the parasitic capacitor).
(2) Since an external resistor and a feedback resistor of a first-stage I/V amplifier are not produced by a common production process, a variation in resistance lacks consistency, whereby the resistance ratio (gain) tends to vary. This imposes limitations to the resistance of the external resistor and the value of the input voltage, thereby making it difficult to achieve a high gain (DC gain).
(3) An input resistor and a switch that switches the electrical connection of the input resistor may be provided in a detection circuit (sensor circuit). The input resistor may be connected to a first-stage I/V amplifier during evaluation, and may be disconnected during normal operation by operating the switch. In this case, since the switch is necessarily inserted in a normal signal input path of the first-stage I/V amplifier, a parasitic capacitor (and impedance) of the switch cannot be disregarded. Since the parasitic capacitor of the switch and the like vary depending on the detection circuit (sensor circuit), it may be impossible to ensure a highly accurate operation of the of first-stage I/V amplifier.